Golf balls fall into three categories according to their structure. First, the three-layered ball consisting of a core of high-density synthetic rubber, a stretched rubber band wound around the core, and a cover. Second, the one-piece ball solidly made of synthetic rubber, with dimples formed on the surface. Third, the two-piece ball made up of a core of synthetic rubber and a cover of ionomer resin or the like. In addition to these three types of balls, there is conceivable the three-piece ball or multi-layered ball consisting of three or more layers, each having different degree of elasticity.
The two-piece ball is improved in durability owing to the covering layer made of ionomer resin ("Surlyn", a product of Du Pont, U.S.A.), which transmits the rebound resilience and rigidity of the core and protects the ball from damage and deformation, thanks to its outstanding strength and hardness.
The core of the two-piece ball is required to be improved in rebound resilience that affects the flight distance and breaking strength that relates to durability. Moreover, the core determines what the player feels when he makes a shot and provides the pleasant sound the ball produces when it is hit.
The core is typically made of polybutadiene rubber covulcanized with an acrylic ester or a metal salt of polymerizable organic acid. The rubber is incorporated with several kinds of fillers so that the ball meets the standards and the ball has the desired flight distance and durability. According to the Rules of the U.S. Professional Golfers' Association, the Rules of the British Professional Golfers' Association, and Japanese Industrial Standard JIS S7005-1955, the diameter is fixed at 42.67 mm (min) and the weight at 45.9 g (max) for American size balls, and the diameter is fixed at 41.15 mm (min) and the weight at 45.9 g (max) for British size balls. Major fillers in use for golf balls are white carbon and zinc oxide. Calcium carbonate, which is used in large quantities for natural rubber, SBR, and other rubbers, is not used in the production of golf balls because it does not impart the required rebound resilience and durability to golf balls.
It has been known that precipitated colloidal calcium carbonate having the BET specific surface area of 20 to 60 square meters per gram (m.sup.2 /g) improves the rebound resilience of butadiene rubber (BR) when incorporated into BR; but it has not been put to practical use because the rubber incorporated with it is low in compression strength. In the meantime, such fine calcium carbonate is produced by calcining limestone at about 1000.degree. C. to give CaO, hydrating CaO to give a suspension of Ca(OH).sub.2, and introducing carbon dioxide gas into the suspension.
The present inventors found that a rubber compound filled with extremely fine calcium carbonate having a BET specific surface area greater than 60 m.sup.2 /g can be advantageously used for making the cores of two-piece golf balls. The cores made of this rubber compound are made into golf balls having compression strength greater than 7000 kilograms of force (kg-f). Golf balls having the core made of the conventional rubber compound filled with white carbon or the above-mentioned precipitated calcium carbonate has compression strength of 2000 to 5000 kg-f. Moreover, the golf ball of this invention has a long flight distance, gives good feeling to the player when hit, and produces a pleasant sound when hit.